An important component for dense wavelength division multiplexing (DWDM) systems is a high-speed, low-voltage modulator, which is usually called an “electro-optic modulator” or “electro-optic device”. It is also known as an EO modulator or EO device. The materials suitable for use in such a modulator should have the nonlinear optical (NLO) property with large electro-optic coefficients (e.g., >50 pm/V) at the desired telecommunication wavelengths. Telecommunication wavelengths are those prescribed by some standard setting body such as the International Telecommunications Union (ITU). Common telecommunication systems use, for example, 1310 nm and bands of 1485-1525 nm, 1525-1562 nm and 1565-1620 nm, as prescribed by the ITU.
The electric polarization of a medium subjected to external electric fields can be written as:Pi(ω)=Pi0+χij(1)(−ω)Ej(ω)+χijk(2)(−ω,ω1,ω2)Ej(ωj(ω1) Ek(ω2)+χijkl(3)(−ω,ω1,ω2,ω3)Ej(ω1)Ek(ω2)E1(ω3)+ . . . where Pi0 is the static polarization and the rest terms are the linear [χij(1)] and nonlinear [χijk(2), χijkl(3), . . . ] susceptibility tensors. The second-order nonlinearity includes second harmonic generation [χijk(−ω,ω1,ω2)] and the linear EO effect (Pockels effect) [χijk(2)(−ω,0,ω2)]. The Pockels effect is the effect of interest for EO modulator device application. It is related to the equivalent molecular hyperpolarizability β. A strong asymmetric ‘push-pull’ molecule with both electron accepting and electron donating groups linked by a conjugated moiety usually shows a finite β value.
The EO effect is not a naturally occurring property in polymers, in contrast to the situation in crystal materials such as lithium niobate. Instead, in polymers it is the product of custom synthesis and an alignment (poling) procedure. The poling process is facilitated by a high ground state dipole moment of an NLO polymer. The induced linear EO coefficient parallel to an applied field is given by:χzzz(2)=Nβ111fω2f02μE/5kT where N is the density of NLO chromophore, β111 is the first hyperpolarizability of the chromophore in the direction of μ, f107 2 and f02 are local field correction factors at frequencies ω and zero respectively, μ is the ground state dipole moment of the chromophore, E is the applied electric field and T is the temperature.
The fact that the dielectric constant of polymers is typically one order of magnitude lower than that in lithium niobate indicates that NLO polymers would be suited to the EO modulator application. Polymers could in principle exceed the performance of current ferroelectric materials for high-frequency EO devices. Additional polymer properties which favor the use of organic polymers in the photonic device applications are their ease and versatility of processing and integrability and synthetic versatility.
To obtain useful properties for an EO polymer, it is desirable to maximize N and the product μβ. The former is determined by the number of active NLO chromophores in the polymer and the latter is a property of the chromophore utilized. Another important requirement is to have a functional group that allows the chromophore to be incorporated into or grafted onto a host polymer. Incorporation or grafting of chromophore into a host polymer is a necessary step towards the device fabrication, as the host polymer material provides a framework that can be processed into a waveguide structure.
A large number of chromophores without functional groups for grafting have been synthesized and some of these exhibit very large macroscopic nonlinearities in guest/host poled polymers. Two NLO chromophores, chromophore-1 reported by Dalton et al (Opt. Lett, 1998, 23, 478) and DEMI reported by Szablewski et al (J. Am. Chem. Soc. 1997, 119, 3144) have the following structures: 
A series of zwitterionic chromophores structurally similar to DEMI but containing the pyridinium and quinolinium moieties has been reported. (Metzger et al, Mol. Cryst. Liq. Cryst. 1984, 107, 133; Ashwell et al J. Chem. Soc., Faraday Trans. 1990, 86(7), 1117.; and Synth. Met. 1991, 43, 3173: Bell et al J. Chem. Soc., Chem. Commun, 1990, 325.) These chromophores have the formulae: 
None of these chromophores has a functional group which permits covalent bonding. Therefore, these chromophores can not be linked to a host polymer via covalent bonding and thus have no practical value for use in an electro-optical device such as electro-optical modulator.